Accumulating evidence suggests that a variety of human cancers originate from the cancer stem cells (CSC) as a result of their ability to hijack self-renewal pathways that are employed by normal embryonic or adult stem cells. WNT, BMI1, Notch, Hedgehog and PTEN pathways are often deregulated in human cancer and these pathways are critical for the self-renewal of stem cells. More recently, microRNAs have also been shown to regulate many important aspects of stem cell biology and tumorigenesis including proliferation, differentiation, apoptosis, invasion and metastasis. However, it still remains poorly understood how these different self-renewal pathways interact with each other to coordinate self-renewal in normal stem cells and how the deregulation of one particular pathway in CSCs affects other pathways during tumorigenesis. Using a genetic screen, we identified a novel function for the tumor suppressor PTEN as a modifier of the microRNA silencing pathway in embryonic stem cells. Loss-of-PTEN mutations have been associated with a wide range of human cancers. Thus, the objectives of this project are to elucidate the molecular mechanisms by which PTEN modulates the efficacy of microRNA silencing machinery in stem cells and to determine the contribution of microRNAs to drive malignant transformation in loss-of-PTEN cancer cells. By characterizing the difference of microRNA silencing mechanisms between normal and PTEN negative stem cells, we hope to develop new therapeutic strategies that specifically target loss-of- PTEN cancer stem cells. PUBLIC HEALTH RELEVANCE: Understanding the similarities and differences between self-renewal mechanisms in normal stem cells and cancer stem cells offers great promises for better treatment of cancer. The tumor suppressor PTEN (phosphatase and tension homolog) pathway has been shown to control normal stem cell maintenance and self-renewal, whereas the loss of PTEN, frequently found in human cancers, promotes the propagation of cancer stem cells and tumor formation. In this study, we will define a novel modifier function of PTEN in the microRNA silencing pathway, and provide a new avenue to distinguish PTEN- negative cancer stem cells versus normal stem cells.